day20

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diff --git a/src/2015/day20/README.md b/src/2015/day20/README.md
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+--- Day 20: Infinite Elves and Infinite Houses ---
+To keep the Elves busy, Santa has them deliver some presents by hand, door-to-door. He sends them down a street with infinite houses numbered sequentially: 1, 2, 3, 4, 5, and so on.
+
+Each Elf is assigned a number, too, and delivers presents to houses based on that number:
+
+The first Elf (number 1) delivers presents to every house: 1, 2, 3, 4, 5, ....
+The second Elf (number 2) delivers presents to every second house: 2, 4, 6, 8, 10, ....
+Elf number 3 delivers presents to every third house: 3, 6, 9, 12, 15, ....
+There are infinitely many Elves, numbered starting with 1. Each Elf delivers presents equal to ten times his or her number at each house.
+
+So, the first nine houses on the street end up like this:
+
+House 1 got 10 presents.
+House 2 got 30 presents.
+House 3 got 40 presents.
+House 4 got 70 presents.
+House 5 got 60 presents.
+House 6 got 120 presents.
+House 7 got 80 presents.
+House 8 got 150 presents.
+House 9 got 130 presents.
+The first house gets 10 presents: it is visited only by Elf 1, which delivers 1 * 10 = 10 presents. The fourth house gets 70 presents, because it is visited by Elves 1, 2, and 4, for a total of 10 + 20 + 40 = 70 presents.
+
+What is the lowest house number of the house to get at least as many presents as the number in your puzzle input?
+
+Your puzzle input is 36000000.
+
diff --git a/src/2015/day20/aoc.cpp b/src/2015/day20/aoc.cpp
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+#include "aoc.h"
+
+namespace aoc2015 {
+
+}
diff --git a/src/2015/day20/aoc.h b/src/2015/day20/aoc.h
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+#pragma once
+#include "common.h"
+
+namespace aoc2015 {
+
+}